PROJECT SUMMARY The rate of obesity is steadily increasing in the United States and around the world, which is a major health concern as obesity is associated with multiple leading causes of death including diabetes and heart disease. Losing weight is often extremely challenging as our bodies naturally reduce energy expenditure in response to weight loss. Adaptive thermogenesis is a sustainable way to maintain body temperature by generating heat from metabolic mechanisms and is one of the components of energy expenditure. Previous studies have shown that reduced thermogenesis causes fat and weight gain, while increased thermogenesis promotes weight and fat loss. Therefore, stimulating adaptive thermogenesis represents a promising approach to treat the obesity epidemic. However, current avenues of research have yet to identify a thermogenic target that could be used to efficaciously promote weight loss. The sodium/iodide symporter (NIS) is the key plasma membrane protein that mediates the sodium-dependent active transport of iodide into the thyroid follicular cells, the first step in the biosynthesis of the thyroid hormones (THs). We have developed a drug-free model of severe hypothyroidism (undetectable THs and increased thyroid-stimulating hormone; TSH) by placing mice that are knockouts for NIS on a low iodide diet. These mice fail to gain weight despite reduced physical activity and similar levels of food intake compared to a model of mild hypothyroidism (wild-type mice on a low iodide diet). In addition, severely hypothyroid mice exhibit significantly increased expression of markers of skeletal muscle adaptive thermogenesis and increased skeletal muscle O2 consumption. On the basis of these preliminary results, I hypothesize that adaptive thermogenesis in the skeletal muscle is stimulated under conditions of severe hypothyroidism, preventing weight gain. To test this hypothesis, I will measure thermogenesis in, and determine the body composition of, euthyroid control mice and two different kinds of severely hypothyroid mice: NIS KO and TSH receptor (TSH-R) KO mice, both on a low-iodide diet (Part a and b). Importantly, using these two different mouse models of severe hypothyroidism will allow me to differentiate between the effects of reduced THs on adaptive thermogenesis and those of increased TSH. Skeletal muscle primary cultures, transport assays in sarcoplasmic reticulum vesicles, and euthyroid TSH-R KO mice will be used to conclusively show that skeletal muscle thermogenesis is stimulated when THs are reduced (Part c). In vitro studies will be performed to determine the mechanisms responsible for hypothyroidism-induced skeletal muscle thermogenesis (Part d). The role of the hypothalamic- pituitary-thyroid axis in skeletal muscle thermogenesis and hypothyroidism-induced thermogenesis are both largely unexplored areas of thermogenic research, and as such will allow us to identify new weight-loss targets that may be used to combat the obesity epidemic.